SAMP-Based Stencil Nano Coatings

Authored By:

Chrys Shea
Shea Engineering Services, Burlington, NJ, USA

Ray Whittier
Vicor VI CHiP Division, Andover, MA, USA

Eric Hanson
Aculon, San Diego, CA, USA

Summary

Stencil nanocoatings have demonstrated significant improvements in numerous aspects of solder paste printing, including print yield, transfer efficiency, print definition and under wipe requirements. By lowering the surface energy of SMT stencils, they reduce flux bleed out around the perimeters of apertures and enable cleaner paste release during stencil-PCB separation.

With several years of commercial success behind the original nanocoating materials, a new generation has been developed that improves upon many of the characteristics of the original formulations. Advancements in durability, detectability and cost boost the overall performance of these flux-repellent stencil treatments. Numerous tests have been performed to characterize stencil nanocoating materials throughout their development cycles and quantify their actual performance in SMT production environments.

These studies have focused on the end results of coating durability and print quality improvements, but have not explored the relationship between flux flow and surface energy modifications on the underside of the stencil. The novel test approach reported in this paper used solder paste treated with UV tracer dye to help image the flow of the flux on the bottom of the stencil.

This paper reviews the test methods and results, and describes the chemical structure of Self Assembling Monolayer Phosphonate (SAMP) nanocoating materials and their influence on the solder paste printing process. The discussion concludes with an overview of related applications of SAMP treatments in the SMT assembly, including printer tooling and accessories, area array/BTC rework stencils and jigs, and placement nozzles.

Conclusions

This experiment focused on visualizing the flow of solder paste flux around stencil apertures on the PCB side of an SMT stencil. UV tracer dye added to solder paste made the difficult-to-see flux highly visible under UV lighting. Using a stencil with one half of a symmetric print area nanocoated and the other half uncoated enabled studying the differences within an experimental system that eliminated most of the external noise associated with comparing different prints on different PCBs or with different stencils.

Flux flow was difficult to see under white light, but immensely obvious under the UV lighting. The nanocoated area of the stencils constrained the flux flow and kept it in or near the apertures; whereas the flux wicked out on the bottom of the stencil on the uncoated area, often bridging two or more apertures with flux. The nanocoated areas also appeared to show less flux smearing after dry wipes, and cleaned more easily with solvent wipes. Overall, the nanocoated area of the stencil stayed cleaner than the uncoated side and cleaned more easily with both wet and dry wipes.

Print definition was improved on the nanocoated side. The microscopy equipment was unable to capture the smallest features in sufficient detail to provide accurate comparisons; however, the crisper print definition was evident on the 0201s and QFNs.

The primary purpose of the nanocoating is to reduce the adhesion of solder paste to the SMT stencil, which it has been proven to do successfully for several years, on all types of stencil materials. Additional applications in circuit assembly operations include:

Printer support tooling, which must be kept clean and free of solder paste to be effective

Printer conveyor rails, which must also be kept clean

Hand tools used in printers and printing areas that are prone to contamination with solder paste

Placement nozzles that pick up solder paste after dropping components

Application of the nanocoating to squeegee blades has been discussed, but can pose some risk because friction between the paste and the blades helps to create the rolling motion needed for proper paste flow.

Initially Published in the IPC Proceedings

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